Theresa May’s hopes of securing the support of the Democratic Unionist Party for her minority government were faltering last night as the Treasury dug in against the costs of a deal.

Some ministers were urging the prime minister to call the Northern Irish party’s bluff as negotiations stalled. Britain’s most senior civil servant, however, told the prime minister that she had little choice but to gain an agreement.

Without a breakthrough soon, the Queen’s Speech, the crucial test of a government’s viability, may have to be put back by a week from the intended date of this Monday to June 26, senior civil service sources said.

Inside Whitehall there were signs that the talks were faltering. DUP sources suggested that the Treasury was dragging its…